Before resetting a switch, the ports should be set to the Disabled state
and the transmit queues should be drained.

Add an helper to explicit that.
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>